This invention pertains to the manufacture of aircraft and more particularly to the optimum aircraft body frame to body skin shear tie installation pattern for body skin/stringer circumferential splices.
The body of an aircraft is typically constructed as a series of spaced circumferential bulkhead members which define the general cross sectional shape of the aircraft, with a series of spaced stringer members running longitudinally with respect to the aircraft body between bulkhead members. The bulkhead members and stringers provide support for the aircraft skin which is formed of a series of separate pieces applied over the various members and joined together with skin splice plates. The skin and splice plates fasten to the bulkhead members via shear ties which transfer load from the aircraft skin into the aircraft body frame. The areas where the skin splice plates are joined to the skin and the shear ties are critical fatigue points subject to potential failure.
Referring now to FIG. 5, which is a view of a bulkhead member and stringer together with splice plate in accordance with the prior art, a bulkhead member 12 defines the general cross sectional shape of the aircraft. Each bulkhead member comprises a series of shear ties 14 formed as flanges having a rectangular inverted "tee" footprint. The bulkhead includes openings 16 between adjacent shear ties 14 such that an individual stringer member 18 passes through each opening. Each opening 16 has a corresponding stringer 18 that passes therethrough; however, for clarity only one stringer is illustrated in FIG. 5. A substantially planar splice plate 20 is located between the bulkhead member shear ties and the aircraft skin and serves to join first skin portion 22 and adjacent second skin portion 24. A series of rivets or other suitable fasteners 26 are provided to join the splice plate and the skin to bulkhead member shear ties 14, thereby ensuring that the skin is securely attached to the plane. Stringer 18 is held to bulkhead member 12 via stringer hanger 28 wherein the stringer hanger 28 attaches to both bulkhead member 12 and stringer 18. An upper portion of stringer hanger 28 is riveted or otherwise attached to the bulkhead member while a lower foot portion 28' is similarly attached to the stringer member 18.
Since the skin splice locations are critical fatigue points, it is desirable to enable easy inspection of such joints so that throughout the life of an aircraft, any potential failures may be identified early and repaired before catastrophic failure occurs. An ideal damage tolerant structure should be provided so that failure initiation and propagation is not concealed, and which is accessible for visual inspection, thereby avoiding the use and resultant expense of non destructive inspection (NDI) methods. The two most likely failure points are illustrated as cracks or failure lines 30 and 30' running along each side of central web 13 of circumferential bulkhead member 12, substantially in line with the rows of fasteners 26 which are nearest the central web. In the construction according to the prior art shown in FIG. 5, visual inspection for potential failure (crack 30 or 30') from within the interior of the aircraft is not possible since the potential failure lines 30 and 30' are hidden from view by shear tie members 14. Also, while the failure line could potentially be visible in area 32 between the shear tie members near openings 16, the placement of stringers 18 prevents such viewing, since each stringer 18 substantially covers the areas 32 between shear tie members 14, blocking the view of the skin splice plate from within the aircraft fuselage. Therefore, in order to adequately inspect for potential failures, expensive non destructive inspection (NDI) methods are required. For example, X-ray or eddy current testing would be necessary to locate such potential failure problems. Such added expensive testing or inspecting makes such inspections less likely to be performed with great frequency or leads to excessive aircraft down time to accomplish the testing.
In efforts to avoid the inspection problems noted hereinabove, previous aircraft designs attempted to make the skin splices fatigue critical in the skin fasteners (e.g. rivets 26) but such a philosophy led to heavy splices (weight reduction is important in aircraft manufacture) and were not successful, since fatigue cracking still occurred in areas inaccessible for visual inspection (i.e. under the shear ties).